This invention relates to the shaping of a membrane, particularly a plastic membrane and more particularly a porous unexpanded polytetrafluoroethylene (PTFE), such as that made by the matrix technique disclosed in U.S. Pat. No. 3,497,256, issued Feb. 24, 1970.
A porous membrane permits passage of material through it proportional to its open pore volume. Also, the larger the surface area of a membrane, the greater is the quantity of material that the membrane can pass. Frequently, a membrane spans a limited area. To increase the effective surface area of the membrane within the confined area, it is conventional to pleat, corrugate or otherwise reshape the membrane to be other than flat in cross section. Thus, a greater functional surface area section of the membrane is used to span the confined area than would be the case were the membrane flat. In addition, in certain applications, the membrane must be shaped, e.g. pleated, corrugated, or the like to generate certain mixing or flow patterns in liquids or gases that flow past one or both surfaces of the membrane.
Various techniques for causing a membrane to be non-flat, shaped, corrugated, etc. are presently practiced. The membrane could be initially extruded through a corrugating die, or molded, or cast in a mold. Dies and molds can be quite expensive. But, it is much less expensive to obtain plastic film in large commercial quantities in flat sheet form than it is to extrude or mold unusually shaped plastic membranes.